1. Field of the Invention
The present invention relates to an imaging lens, and more particularly to an imaging lens which is suitable for installation into an image input device of a portable telephone or personal computer, a digital camera, a CCD camera used for monitoring purposes, a surveying device, or similar which uses a CCD or CMOS as an imaging device.
2. Description of Related Art
In such an imaging lens, the optical length, which is defined as the distance from the entrance surface on the object side of the imaging lens to the image surface (the image-forming surface of a CCD or the like), must be short. An imaging lens having a short optical length will occasionally be referred to as a compact lens below.
Taking a portable telephone as an example, the optical length must at least be shorter than the thickness of the portable telephone body. Meanwhile, the back focus, which is defined as the distance from the exit surface on the image side of the imaging lens to the image surface, is preferably as long as possible. This is due to the need to insert components such as a filter between the imaging lens and the image surface.
Apart from the above, a lens for image pickup is required to be corrected such that various aberrations are reduced by a sufficient amount that distortion of the image is not visually perceptible, and as required by the integration density of the imaging elements (also called “pixels”). An image for which various aberrations are satisfactorily corrected may be called a “satisfactory image”.
The prior art discloses imaging lenses with a three-layer structure which are used in imaging devices using a solid-state imaging device such as a CCD or CMOS installed in portable computers, video telephones, or the like. These lenses secure a wide viewing angle, and are compact and lightweight.
First, an imaging lens capable of obtaining images with satisfactorily corrected aberration while securing a wide viewing angle has been disclosed as a first three-layer lens (see Japanese Unexamined Patent Application Publication No. 2001-075006, for example).
In this imaging lens, however, the refractive power of the three lenses, constituted by first, second, and third lenses, that are arrayed in succession from the object side, is positive in the first lens, negative in the second lens, and positive in the third lens. Hence this imaging lens has a constitution in which the distance (optical length) from the surface of the first lens on the object side to the image surface cannot be shortened. Further, in this imaging lens, a diaphragm is disposed on the object side surface of the first lens, and hence the effective diameter of the third lens cannot be reduced. As a result, a compact lens cannot be produced.
Imaging lenses in which aberration is satisfactorily corrected and a short focus is realized while securing a wide viewing angle have been respectively disclosed as second through fourth three-layer imaging lenses (see Japanese Unexamined Patent Application Publication No. 2003-149548, Japanese Unexamined Patent Application Publication No. 2002-221659, and Japanese Unexamined Patent Application Publication No. 2002-244030, for example).
However, similarly to the imaging lens described above, the refractive power of the three lenses of these imaging lenses, constituted by first, second, and third lenses arranged in succession from the object side, is positive in the first lens, negative in the second lens, and positive in the third lens. Hence, although these imaging lenses are set with a short combined focal length as an imaging lens, the back focus is long, and thus the optical length is too long. In addition, these imaging lenses use glass materials, and are therefore expensive.
An imaging lens which uses aspheric lenses and is reduced in size by appropriately setting power distribution and surface shape has been disclosed as a fifth three-layer imaging lens (for example, Japanese Unexamined Patent Application Publication No. 2003-149545).
However, the refractive power of the three lenses of this imaging lens, constituted by first, second, and third lenses arranged in succession from the object side, is negative in the first lens, positive in the second lens, and negative in the third lens. As a result, the imaging lens has a long optical length. In addition, the lenses of the imaging lens use glass materials, and are therefore expensive.
A lens in which a pair of meniscus lenses whose concave surfaces face each other are constituted by plastic lenses each having at least one aspheric surface, and in which the entire lens system has a three-layer structure, has been disclosed as a sixth three-layer imaging lens. This lens achieves compactness and low cost, and is capable of suppressing focus movement due to temperature change with ease (see Japanese Unexamined Patent Application Publication No. H10-301022, for example).
However, the refractive power of the three lenses in this imaging lens, which are arranged as first, second, and third lenses in succession from the object side, is weak in the first lens, weak in the second lens, and positive in the third lens. Hence the refractive power of the first lens and second lens in this imaging lens cannot be fully compensated for by the third lens alone. As a result, the back focus lengthens, causing an increase in the optical length. Furthermore, the third lens of this imaging lens uses a glass material, and hence cost reduction is incomplete.
A telephoto-type imaging lens in which the entire lens system is divided into front and rear groups, the front group having a positive refractive power and the rear group having a negative refractive power, has been disclosed as a seventh three-layer lens. This imaging lens has a short optical length and is low cost (see Japanese Unexamined Patent Application Publication No. H10-301021, for example).
However, the refractive power of the three lenses in this imaging lens, which are arranged as first, second, and third lenses in succession from the object side, is negative in the first lens, positive in the second lens, and negative in the third lens, and the distance between the second lens and third lens is wide. As a result, the optical length of the imaging lens is long, and the aperture of the third lens widens. This is unsuitable for installation in image input devices of portable telephones or personal computers, digital cameras, CCD cameras used for monitoring purposes, surveying devices, and so on.
It is therefore an object of the present invention to provide an imaging lens which is suitable for installation in a camera using a CCD or CMOS as an imaging device, which has a short optical length, and which is thus capable of obtaining satisfactory images.
A further object is to provide an imaging lens in which all of the (three) lenses constituting the imaging lens are made of plastic materials to thereby reduce cost and weight.
Here, “plastic materials” refers to high polymeric substances which may molded by being subjected to plastic deformation through application of heat, pressure, or both and thereby formed into lenses, and which are transparent to visible light.